Semiconductors, such as transistors and diodes, are sometimes fabricated in a semiconductor package wherein the mechanical mounting is combined with an electrical connection. An example of a top view 102 and a bottom view 104 of such a package, which is sometimes referred to as an exposed tab package, is shown in FIG. 1. Exposed tab semiconductor packages are sometimes employed instead of encapsulated packages, because exposed tab packages may exhibit better heat transfer, power handling, and current handling capabilities than encapsulated semiconductor packages. Exposed tab semiconductor packages are available in at least two configurations—one configuration, as shown in FIG. 1, designed to be mounted using mechanical hardware (such as a screw, a nut, and/or an insulator) and another configuration, as illustrated in FIG. 2, designed to be solder-mounted to a solderable surface of a printed circuit board (PCB) or another solderable surface. Each exposed tab package configuration has its advantages and disadvantages.
Some benefits of employing the hardware-mounted exposed tab semiconductor package include that: (1) the PCB area and copper pours required for the device are minimized; (2) the mechanical mounting is rugged and secure; (3) the thermal conductivity from the semiconductor to its mounting and/or heat sink is significantly improved; and (4) the mechanical hardware can provide electrically isolated devices.
Some drawbacks of employing the hardware-mounted exposed tab semiconductor package include: (1) the additional mounting hardware required to secure the devices; (2) the manual labor that may be required to install the mechanical hardware may incur additional component and assembly costs; (3) the manual nature of the assembly can be error prone; (4) when a common heat sink is used for multiple semiconductor devices, the devices that have different voltages on their respective exposed tabs must be electrically isolated from the heat sink and/or from each other; and (5) device repair and/or replacement can be time consuming.
Some benefits of employing the solder-mounted exposed tab semiconductor package include: (1) the thermal conductivity from the semiconductor device to the PCB and its associated mounting and/or heat sink is significantly improved; (2) the electrical circuit parasitics can be reduced due to shorter and more direct electrical connection between the device and the solderable surface; (3) the generation of and susceptibility to electromagnetic interference by the device can be reduced by virtue of the reduced electrical circuit parasitics; (4) the ability to use solder, which can be inexpensive and machine-installed, as the mechanical mounting means; and (5) the flexibility in being able to pick-and-place-mount the device in many possible PCB locations.
When the solder-mounted exposed tab semiconductor package is employed, heat sinking is sometimes accomplished by using large circuit board copper planes, or by affixing heat sinks atop the semiconductors, as shown in FIG. 2. However, a number of challenges arise because the mechanical mounting surface of the solder-mounted exposed tab package also serves as the electrical connection to the exposed tab. For instance, because the heat sinks shown in FIG. 2 are also electrically connected to the respective semiconductors, the voltage potential of the large heat sinks can reach a lethal level and thus pose a danger to service personnel. Additionally, because connecting multiple semiconductors to a common heat sink would result in electrically shorting the semiconductors to each other, each semiconductor requires its own separate heat sink. Moreover, employing large circuit board copper planes or large external heat sinks electrically connected to semiconductors to conduct heat away from the semiconductors can have deleterious effects on the performance of the electrical circuit, such as increased parasitic capacitance, and/or increased electromagnetic interference (EMI) generation and/or susceptibility.
Challenges that arise from employing a common heat sink for multiple semiconductor devices include that devices having different voltages on the exposed tab must be electrically isolated from the heat sink and/or each other, and additional mounting hardware may be required to secure the heat sink. Additionally, the need to repair and/or replace one or more semiconductor devices that share a heat sink with other semiconductor devices may require the removal of the external heat sinks, such as those shown in FIG. 2, which is time consuming.
The solder-mounted exposed tab semiconductor package is the preferred device package for many applications at least in part because of its pick-and-place mounting capability, performance enhancements, overall lower cost, and other benefits. However due to the drawbacks listed above, and others, there is a need for an improved system and method for thermal management of electronic devices.